Hello, i think i have the same problem he has so here is some screenshots, one with the initial Mirror Modifier position, and one after moving it up. As you can see, it doesn't solve the "problem" it solved in the video and it move the begining of the mesh a little above the origin of the Skin Modifier.
Nevermind, i ended up restarting from scratch and it works. Looking back at my screenshots, i think the issue was related to the fact that my "line" was not a perfectly straight on Z.
